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The Case of Glass Tolerances


Dear Glass Detective,
What are the tolerances for ¼” glass as far as flatness or being level is concerned?
Thank you,
Brent R.


Dear Brent,
Thank you for contacting the Glass Detective with your questions concerning the tolerances or “flatness” for what you are referring to as ¼” glass. At first glance, your question might seem a little too specific or technical in nature for a consumer-based information site open to the public, yet it is a question that we have received before. So we felt that it was a good time to put out a more thorough or comprehensive answer than you might have expected. Okay, so here we go:

Basically, the industry operates or complies, if you will, under standards produced by a number of organizations, including one known as ASTM International. ASTM is an acronym for the American Society for Testing and Materials. Though, the organization is known today as ASTM International.

This organization came into existence in 1898 when a group of scientists and engineers came together to develop a standard for the steel being used to fabricate railroad rails. Now, 120 years down the road, this not-for-profit group has 30,000 members around the world. In the latest report, the ASTM team has 12,575 standards accepted and used worldwide. And yes, the glass industry recognizes and uses dozens of standards set forth by ASTM International.

One more chunk of information on this—in 1965, the U.S. Government passed the National Technology Transfer and Advancement Act which requires the federal government to use privately developed consensus standards whenever possible. In other words, the standards that many industries had been using were now going to be formally recognized as federal standards. My first thought is that this saved taxpayers a lot of money. And I guess it might have also helped to keep politics out of the standards developing process. Now back to glass.

The federal standard that applies to glass is DD-G-451d. It incorporates standards developed by ASTM for all things glass. This standard deals with everything, including acceptable types and sizes of defects and scratches, manufacturing tolerances, and other concerns of this nature. It even has standards for the quality level for glass that will be made into mirrors.

I’ve likely given you more background information than you expected or wanted, but I thought it was worthwhile to provide this for reference purposes. As to your ¼” thick glass, I turn to the table in my copy of the federal standard and read that the acceptable tolerances for thickness for a piece of ¼” glass (technically listed as .23”) has an acceptable range of .219” to .244”. Thus, some variation in thickness is acceptable, and of course, that variance could exist in a single given piece of glass.

HOWEVER…and this is a big “HOWEVER,” I think within a given size range, you can have a high level of confidence in the “flatness” of a piece of ¼” float glass because the process by which this glass is made renders it very flat. You see, the glass is liquid as it flows onto a molten bed of tin. The glass is then going to cool slowly (anneal) while it is solidifying. Liquids always seek (in a natural state) their own level. To some extent, the glass is self-leveling.

Quality control techniques used by the manufacturers look for distortions of any and all kinds during the manufacturing process. The glass coming out of the North American glass plants is of extremely high quality. To verify the flatness of a piece of glass, you could also have it analyzed using equipment that I’m sure can satisfy your requirements if you need to go to that extent. Not knowing what you are looking for, how precise you need to be, or how deeply you want to go into this, I can also suggest that you go online and search the internet for ASTM C1036-06 and ASTM C1036-21 (Standard Specification for Flat Glass).

I hope that this information is of some value to you. I also want to wish you good fortune with your project and encourage you to use all proper safety precautions when working with glass. You may want to take a quick look at some of our other consumer information blogs, which talk about safety glass types and uses. And last but certainly not least, I want to thank you again for contacting with your information request.

-The Glass Detective

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Lyle Hill

Lyle Hill has been in the glass and metal industry for more than 40 years. In this time he has managed glass retail, contract glazing, mirror, architectural window, window film, and automotive glass businesses throughout America. He obtained an MBA from IIT with a focus on Technology and Engineering Management. Hill is also a columnist for glass industry trade magazines and often called the “face” of the glass industry. He has also authored books including “The Broken Tomato and Other Business Parables,” which is available through Amazon. Find out more about Lyle on Linkedin.

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